The ZnO materials of our case formed in the zinc-rich (oxygen- deficient) condition, especially for the positions near the source material and far from the opened en[r]
Trang 1SELF-CATALYTIC FABRICATION OF
ZnO MICRO- AND NANO-TETRAPODS
N go X u a n D a i, N g u y en T hi T hu c H ien , N g u y e n N g o c L ong
D epartm ent o f Physics, College o f Science, VN U
Abstract: Tetrapod-like zinc oxide (ZnO) micro and nanorods were produced on oxidized silicon substrates by thermal evaporation of Zn/ZnO powder mixture The synthesis procedures were carried out at 1050°c in air, and there is no arry catalyst used The fabricated ZnO tetrapods were quite uniform, high yield, good crystallinity and especially had very strong u v and green photoluminescence at room temperature The forming and photoluminescence mechanisms of the tetrapods were considered
1 In tro d u c tio n
ZnO has been realized as an excellent functional material for many fields of application Novel superstructures of ZnO, such as nano comb-, propeller-, tetrapod-, teeth like shapes [1-5], promise a great deal of interesting physical property, and can be used as special building blocks, components for micro/nanosystems Recently, in the world, many attempts have been made to synthesize, characterize and apply ZnO tetrapod-like structures [2-5] Authors of [2] found that the sintering temperature of their ZnO-glass varistors were evidently lowered because of the higher activity of tetrapod (TP) ZnO nanopowders Low-field electron emission and the technological usefulness of ZnO TPs synthesized by rapid heating of zinc pellet was reported in [3] Room-temperature photoluminescence (PL) of ZnO TPs prepared by oxidation of zinc powders [4] or evaporation of ZnC 03 powder [5] was clearly observed and it showed the potential ability for fabricating light-emitting devices in nanoscale optoelectronics For the nice application prospects, the synthesis procedures, forming mechanisms and physical properties of ZnO TPs are continuatively studied and developed.
In this paper, we report our efficient route for synthesis ZnO TPs and the beginning discussion on their structural, photoluminescence properties.
2 E x p e rim e n ta l
Our synthesis method is based on a vapor
transport process and a simple experiment setup shown
in figure 1 The source material for evaporation is a
mixture of high purity zinc metal (Zn) and zinc oxide
(ZnO) powders (molar ratio 2:1) The source material
and the SiO^Si substrates were placed inside a quartz
tube, the source material was located at the closed end
while the substrates were arranged toward the opened
end of the tube The quartz tube was inserted to a
horizontal furnace in air ambient The temperature of the source material was ~1050°c, of
Furnace quarzttube
source material substrates
F ig l Experiment setup for synthesis of ZnO tetrapods
Trang 2Self-catalytic fabrication of 3 9
the substrates was in range 850-:-950°C The temperature was kept for 1.5 hour, and then naturally cooled down to room temperature After the experiment, white fluffy products of mass yield were found on the substrates and the quartz tube The products covered on the substrates were collected for characterization Morphology and crystal structure of the products were characterized bv scanning electron microscopy (SEM) (JEOL JSM 5410 LV) and X-ray diffractometer (Brucker D5005) Photoluminescence of the samples were recorded by a spectrofluorometer (FL3-22 Jobin-Yvon).
3 R e su lts a n d d is c u s s io n
X-ray diffraction (XRD) patterns of the
synthesized products show characteristic diffraction
peaks of hexagonal wurzite ZnO crystal phase, there
is no any diffraction signal of impurity (fig.2).
The typical SEM image shown in fig.3
illustrates the tptrapod-like shape of the synthesized
ZnO products Each tetrapod is composed of four ZnO
micro/nanorods - legs of tetrapods - with the diameter
o f about 500-7-2000 nm and length of several
micrometers, this sizes depend on the location of the
substrate in the quartz tube of the synthesis process.
All the legs o f the TPs have smooth surface and
particularly have hexagon end planes, as can be
seen in the SEM image (fig 3).
Hexagonal wurzite cell of ZnO presents a
polar oxide (or zinc) {001] plane and an electrically-
neutral non-polar [100] plane, as shown in the inset
picture of figure 2 The polar plane is metastable
and thus favors a fast growth rate, while the non
polar face has higher stability Ị6Ị We believe that in
our synthesis conditions ZnO nuclei formed in the
typical hexagonal wurzite structure and fast grew
up layer-by-layer along the c-axis (<0001>),
therefore resulted in hexagonal shape, end planes
and single crystalline of the legs of the ZnO TPs.
Both vapor-liquid-solid (VLS) and vapor-solid
synthesis procedures because at the synthesis temperature (850-950"C), ZnO solids, ZnOx (x<l), Zn liquids could exist together and play as the self-catalysts for the growth of ZnO micro/nanorods (the legs of TPs) [1, 5], However, the v s mechanism may dominate due to that Zn and ZnO, liquid droplets (with boiling point of 907°C) easily transit to gaseous state
at the synthesis temperature The explanation for the forming of the tetrapod-like morphology is still an exciting problem for worldwide research May be, the volcanic oxidation of Zn powder plays an important role leading to the growth of the ZnO TPs Four
2 theta degree Fig.2 XRD pattern of ZnO tetrapod products ; The inset is the hexagonal unit cell of ZnO [6]
Fig 3 A typical SEM image of the synthesized ZnO products (VS) processes could occur in our
Trang 34 0 Ngo Xuan Dai, Nguyen Thi Thuc Hien, Nguyen Ngoc Long
Room-temperature photoluminescence spectra of
the ZnO TPs excited with a 335 nm light (from a 450W
Xenon lamp) were shown in Fig 4 Very inten se
emission peaks at 388 nm (UV) and 495 nm (green)
were observed in all the synthesized samples The first
narrow peak at 388 nm can be attributed to excitonic
recombination This u v peak is red shift in comparison
with the 380 nm peak of usual cases of ZnO The broad
peak at 495 nm is mainly due to oxygen vacancies
(inducing deep levels) in the ZnO crystals [7] The ZnO
materials of our case formed in the zinc-rich (oxygen-
deficient) condition, especially for the positions near the
source material and far from the opened end (air
ambient) of the synthesis quartz tube (fig 1) The
further distance from the opened end of the tube, the
higher probability of forming oxygen vacancies, thus the
green emission increases and the ƯV emission is
partially quenched (fig 4) The representative excitation
spectrum of the 495 nm emission peak (fig.5) indicates
the optical semiconductor nature of the ZnO TPs with
the clea r excito nic absoi'ption peak a t 381 nm G ood
excitonic optical transitions can reveal the high purity
and good crystallinity of the ZnO products.
4 S um m ary
We have fabricated tetrapod-like ZnO micro/nanorods by a sim ple experiment setup The synthesis process, forming mechanism and physical properties of the ZnO tetrapods are still the land for active researches in coming time Very high intense ultra-violet and green photoluminescence at room temperature of our synthesized ZnO tetrapods exposes the potential for fabricating micro and nanoscale light-em itting devices.
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381 nm
wavelength nm
Fig.5 Excitation spectrum of the synthesized ZnO TPs
wavelength nm
Fig.4 P L spectra o f the ZnO TP s formed near (solid line) and far from (dotted line) the opened end
of the quartz tube